High Quality Hot-wire Microcrystalline Silicon for Efficient Single and Multijunction N-i-p Solar Cells
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0910-A26-03
High Quality Hot-wire Microcrystalline Silicon for Efficient Single and Multijunction N-i-p Solar Cells Robert L. Stolk, Hongbo Li, Ronald H. Franken, Karine H.M. Van der Werf, Jatindra K. Rath, and Ruud E.I. Schropp Utrecht University, Faculty of Science, Department of Physics and Astronomy, SID-Physics of Devices, Princetonlaan 4, Utrecht, 3584 CB, Netherlands
ABSTRACT In this paper, the potential of hot-wire chemical vapor-deposited (HWCVD) microcrystalline silicon (µc-Si) for use in solar cells is explored. Incorporation of the material in the currentlimiting bottom cell of two tandem cells on plain stainless steel resulted in FF values as high as 0.77, which is much higher than the highest single junction FF. A combination of experiments, calculations and computer simulations was employed to identify causes for the observed high tandem cell FF values. Both the light intensity and the spectral composition of the bottom cell illumination in a tandem were found to contribute to an increase of the bottom cell FF. The fact that the operational voltage of a tandem cell is higher than that of the current-limiting subcell, was calculated to lead to a tandem FF that can be far higher than that of the limiting cell. Computer simulations with the AMPS computer code show that the current mismatch in a tandem cell reduces the recombination in the current-limiting cell, possibly by slightly enhancing the internal field of that cell. Use of a 1.5 µm µc-Si:H hot-wire deposited absorber layer in a single junction cell on a textured back reflector yielded a Voc, FF and Jsc of 0.543 V, 0.656 and 23.60 mA/cm2, respectively, which combine to an 8.4 % record efficiency for µc-Si single junction n-i-p cells with a hot-wire intrinsic layer. INTRODUCTION Intrinsic microcrystalline silicon (µc-Si) is very suitable for use in highly efficient single and multijunction thin film silicon solar cells. For single junction µc-Si p-i-n cells, efficiencies around 10 % have been reported [1,2], whereas tandem and triple junction cells with a µc-Si absorber in the bottom cell have yielded 14.1 % [3] and 15.0 % [4], respectively. Using hot-wire chemical vapor deposition (HWCVD), we have developed µc-Si material close to the amorphous-to-microcrystalline phase transition, which has a high optoelectronic quality [5,6] and is an intended bottom cell absorber layer in n-i-p-configured triple junction solar cells. Single junction n-i-p cells on plain stainless steel with this material as an intrinsic layer, showed Voc and FF values as high as 0.54 V and 0.72 [7], respectively. For bottom-cell limited tandem cells with this material as their bottom cell intrinsic layer, we observe FF values that are larger than the best single junction value [7,8]. In this paper, we present possible causes for this observation, making use of experiments, calculations and computer simulations. Furthermore, we report on the current status of our n-i-p µc-Si single junction cells on a textured back reflector.
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